The present invention relates to a thrust measurement system for small planing watercrafts, which is designed to measure the thrust of a small planing watercraft under the condition that the watercraft is received in a water pool in a floating condition.
Japanese Patent Publication No. (SHO) 62-6173 discloses a method of measuring the thrust of a watercraft propulsion unit. The disclosed thrust measuring method, as illustrated here in FIG. 8, includes two strain gauge units 102, 102 attached by bonding to an inner surface 101a of a casing 101 of the propulsion unit 100 in such a manner that the strain gauge units 102, 102 are disposed in diametrically opposite relation to each other. Each strain gauge unit 102 has two strain gauge elements (not shown) which are connected with two strain gauge elements of another strain gauge unit 102 so as to form a bridge in a manner known per se.
During thrust measurement operation, the propulsion unit 100 is driven to rotate a screw-propeller 104, and the amount of strain occurring in the casing 101 due to rotation of the screw-propeller 104 is measured by the bridge of the strain gauge elements. As is well known, there is a certain relationship established between the thrust of the propulsion unit 100 and the amount of strain of the casing 101. Accordingly, by thus measuring the amount of strain of the casing 101, a thrust of the propulsion unit 100 can be readily determined.
The conventional thrust measuring method discussed above is labor intensive because the strain gauge units 102. 102 must be attached to and removed from the casing 101 each time the thrust measurement for one propulsion unit 100 is achieved. Furthermore, the conventional thrust measuring method is limited for use with a propulsion unit alone that is not installed in the hull of a watercraft. There has been a desire to realize a thrust measurement system which is capable of measuring the thrust of a propulsion unit as it is installed in the watercraft.
It is, accordingly, an object of the present invention to provide a thrust measurement system for small planing watercrafts, which is labor-saving, easy to conduct and capable of measuring the thrust of a propulsion unit as it is installed in the hull of a watercraft.
According to the present invention, there is provided a thrust measurement system for watercrafts, comprising a water pool for receiving therein a watercraft in a floating condition, a bow holding apparatus disposed on an edge of the water pool for holding therein a bow of the watercraft during thrust measurement operation, a thrust measurement device mounted in the bow holding apparatus for measuring a thrust of the watercraft, and anchoring means spanning between the edge of the water pool and a hull of the watercraft for anchoring the watercraft in position against lateral swinging movement about the bow.
With the thrust measurement system thus arranged, the thrust of a propulsion unit used for propelling a watercraft can be measured under the condition that the propulsion unit is installed in the watercraft. This eliminates the need for a laborious manual operation which is employed in the conventional thrust measurement process of FIG. 8 for the purpose of attaching or detaching the strain gauge units relative to a casing of the propulsion unit.
In one preferred form of the present invention, the bow holding apparatus includes a plurality of support columns, a carrier member slidably mounted on the support columns for vertical movement along the support columns, the carrier member carrying thereon the thrust measurement device, a cylinder actuator for moving the carrier member upward or downward along the support columns, a slide member slidably mounted on the carrier member for horizontal reciprocating movement in a direction toward and away from the water pool, a bow holding mechanism for holding therein the bow of the watercraft, the bow holding mechanism being connected to the slide member and held in contact with a probe of the thrust measurement device, and a balancing device that counterbalances to the weights of the bow holding mechanism, thrust measurement device, slide member and carrier member such that the bow holding mechanism, thrust measurement device, slide member and carrier member together form a floating structure that can move freely in a vertical plane.
By virtue of the floating structure of the bow holding apparatus, the bow of the watercraft is allowed to freely move up and down as the watercraft moves to drift in synchronism with heaving of the pool water surface.
Preferably, the bow holding mechanism has a bow hold member having a recessed portion of a configuration snugly receptive of the bow of the watercraft, a vertically disposed thrust board connected to the slide member for movement in unison with the slide member and held in contact with the probe of the thrust measurement device such that a movement of the slide member in a direction away from the water pool causes the thrust board to push the probe of the thrust measurement device, and a ball joint articulately interconnecting the bow hold member and the thrust board. interconnecting the bow hold member and the thrust board.
With this articulate coupling using the ball joint, the bow hold member can take various postures relative to the thrust board, which may occur when the watercraft undergoes pitching, rolling, steering and accelerating.
The bow holding mechanism may further have a plurality of compression coil springs acting between the bow hold member and the thrust board and arranged at regular intervals around an axis of the ball joint. Furthermore, the bow holding mechanism may also have adjustable spring retainers each supporting one end of a respective one of the compression coil springs, the adjustable spring retainers being movably mounted on one of the bow hold member and the thrust board and separately movable in a direction to change a preload applied to the individual compression springs. Preferably, the adjustable spring retainers each comprise a stepped round bar having a thread-free small-diameter portion, a threaded large-diameter portion, and an annular flange separating the small-diameter portion and the large-diameter portion, the annular flange supporting thereon one end of each compression coil spring with the thread-free small-diameter portion received in an internal space of the compression coil spring, and the threaded large-diameter portion being threaded through an internally threaded portion of the thrust board. The bow hold member may have a plurality of recessed portions arranged around the axis of the ball joint and each receiving therein the opposite end of a respective one of the compression coil springs.
The anchoring means preferably comprises a first wire rope having one end connected to a first edge portion of the water pool and the other end adapted to be connected to a central portion of a stern of the watercraft, a second wire rope having one end connected to a second edge portion of the water pool diametrically opposite the first edge portion and the other end adapted to be connected to the central portion of the stern of the watercraft, and a third wire rope having one end connected to a third edge portion of the water pool diametrically opposite an edge portion where the bow holding apparatus is disposed, the other end of the third wire rope being adapted to be connected to the center of the stern of the watercraft. It is further preferable that the first, second and third wire ropes each have two hooks at the one end and the opposite end thereof, and means for adjusting the length of the wire rope.
With the wire ropes thus arranged, the stern of the watercraft is anchored in position against lateral oscillation about the bow being held in the bow holding apparatus. The wire ropes permit vertical movement of the stern during thrust measurement operation.